In a small bowl combine buttermilk and pumpkin. Whisk together until incorporated.
Bake for 18-26 minutes. The cake is done when an inserted toothpick comes out mostly clean.
Add heavy cream to a stand mixer and whip on med to medium-high speed until soft peaks. (usually about 2-3 minutes)
While the cream is mixing place pumpkin puree, cinnamon, and all-spice in a bowl. Stir until combined.
Once the cream is at soft peak stage, add powder sugar in a 1 cup at a time. After one cup, turn off mixer. Gently fold in pumpkin puree mixture to cream. If the consistency is not firm enough, add more powder sugar, about a 1/4 cup at a time until you reach desired consistency.